An internal combustion engine for a vehicle is provided with a variable valve timing controller which adjusts a valve timing (a valve opening timing and a valve closing timing) of an intake valve and an exhaust valve in order to improve its output and fuel economy and to reduce emission. The variable valve timing controller adjusts a rotation phase of a camshaft relative to a crankshaft so that valve timings of the intake valve and the exhaust valve are varied.
Also, gasoline, alcohol, and mixed fuel of gasoline and alcohol are used as fuel for an internal combustion engine in order to reduce CO2 emissions and utilize petroleum substitute fuel. When the mixed fuel of which alcohol concentration has varied is supplied to a fuel tank, the alcohol concentration of the fuel in the fuel tank is also varied.
JP-05-1574A shows a valve timing controller in which an alcohol concentration in the fuel is detected by an alcohol concentration sensor and the valve timing is adjusted according to the detected alcohol concentration.
Also, JP-11-2141A (U.S. Pat. No. 6,094,974) shows a diagnosing apparatus and method. In this apparatus and method, when a specified time period has elapsed from a time when a target camshaft phase (a target valve timing) was varied, it is determined whether the variable valve timing controller has a defect based on whether a difference between the target camshaft phase and an actual camshaft phase is greater than a specified value.
When the engine has been stopped for a long time period, an engine temperature becomes lower than or equal to an ambient temperature. In such a condition, a part of fuel injected by an injector flows down along an inner surface of a cylinder toward an oil pan. This fuel is mixed with a lubricant (engine oil) in the oil pan, so that the lubricant is diluted with the fuel. Especially, when the alcohol concentration of the fuel becomes high, the fuel quantity mixed with the lubricant (fuel diluting quantity) tends to increase.
The variable valve timing controller is operated with the lubricant. As shown in FIG. 2, when the fuel diluting quantity is increased, a viscosity of the lubricant is varied, so that an operating characteristic of the variable valve timing controller is varied. The operating characteristic of the variable valve timing controller represents a relationship between a command value and a response speed of the valve timing controller. As shown in FIGS. 3A and 3B, when the fuel diluting quantity is increased, a responsiveness of the valve timing controller is varied and a normal responsiveness range is also varied. The normal responsiveness range represents a range where the responsiveness of the valve timing controller is determined as normal. Thus, if it is determined whether the responsiveness of the valve timing controller is normal or abnormal based on a defect determination value in a case that the fuel diluting quantity is large as well as in a case that the fuel diluting quantity is small, it is likely that a computer erroneously determines that the responsiveness of the valve timing controller is faulty even though the variable valve timing controller has no defect. The condition where the responsiveness of the vale timing controller is varied due to an increase in the fuel diluting quantity may be erroneously determined as the condition where the valve timing controller is faulty.